Friction stir welding (FSW) may be used to join two sheets of metal along a weld line without the use of welding filler materials. The weld is created by a combination of frictional heating of the metal by a rotating tool, and mechanical deformation of the weld using a rotating tool. These tools may comprise a retractable pin tool rotatable within an annually shaped shoulder tool. The pin tool and shoulder tool may be connected to concentric spindles which are respectively driven by separate motors. These motors are typically mounted on or near a spindle head and are laterally offset from the spindle axes. The ends of the concentric spindles are coupled with the motors either through gear sets or drive belts.
The spindle head construction described above may not be suitable for some applications due to the bulk of the spindle head caused by the laterally offset motors. For example, FSW machines on which the spindle head is mounted may have structural elements that interfere with the laterally offset motors as the spindle head is moved along multiple axes during a welding operation.
Accordingly, there is a need for a FSW apparatus having a compact spindle head which overcomes the problems discussed above. Embodiments of the disclosure are intended to satisfy this need.